Arc diffractometer

ABSTRACT

An x-ray diffractometer has an x-ray detector and an x-ray source mounted on vehicles traveling along an arc-shaped track around a sample. The vehicles move independently. Alternatively, the vehicles can move independently on separate parallel tracks.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation of application Ser. No. 08/828,466 filed Mar. 28,1997.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to the field of x-ray diffractometers.

2. Related Art

In x-ray diffractometry, it is desired to measure diffracted x-radiationfrom a sample. Incident radiation is provided within a measurement planeat an incident angle, omega. Diffracted radiation is observed in themeasurement plane at an output angle, referred to herein as 2theta. Thevalues of omega and 2theta depend on the nature of the sample which isto be subjected to the x-radiation.

In the prior art goniometer-based system, see FIG. 1, the source 101 andthe detector 102 were carried on arms 103 and 104 attached like thehands of a clock to concentric axles. The source and detector protrudedforward from the arms to effect measurements on a sample carried by aplatform disposed in front of the concentric axles. In the prior artsystem, large samples could not be placed on the platform 105, withouthitting the arms. Large samples are currently thought of as being asmuch as 25 cm, though in the future even larger samples might well needto be subjected to x-ray diffraction. The prior art system could not bereadily enlarged to accommodate large samples because the longer armsand larger forward extending protrusions for carrying the source anddetector were too susceptible to vibration and bending to allow accuratemeasurements.

SUMMARY OF THE INVENTION

In the x-ray diffractometer of the invention, an x-ray source is movedin an omega vehicle along an arc-shaped track. The x-ray detector iscarried on a track in a 2theta vehicle which moves independently fromthe first vehicle. Optionally, the vehicles can be on the same track.This structure is mechanically stable when enlarged and allows fullrange of motion for the detector and source.

GB 1390710 shows an x-ray diffraction instrument in which an x-raysource travels on a trolley along an arc-shaped track. X-ray detectorstravel along with the x-ray source on the same trolley. The detectorsare placed along a second arc-shaped track attached to the trolleycarrying the source. EP 044492 shows a similar structure. While the arcsof these patents bear a superficial resemblance to the track system ofthe invention, in fact they cannot be used for general x-ray diffractionmeasurements. In the systems of these patents, 2theta is dependent onomega because the detector tracks ride on the trolley bearing thesource. As a result these two-track systems are incapable of sufficientangle difference between the source and the detector for generalizedx-ray diffraction measurements. These prior systems can only be used forstress measurements. Also, the dependent track structure would becomemechanically unstable if enlarged sufficiently to be used with largesamples.

BRIEF DESCRIPTION OF THE DRAWING

The invention will now be described by way of non-limitative examplewith to the following figures:

FIG. 1 shows a prior art goniometer-based system

FIG. 2 shows a system in accordance with the invention.

FIG. 3 shows a side view of the system of FIG. 2, showing the chi tiltmechanism.

FIG. 4 shows a variable radius alternate embodiment of the invention.

FIG. 5 shows a multiple track alternate embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 2 shows the system of the invention.

The sample rests on a platform 201. The platform rests on a support 202which has actuators for effecting xyz translation and phi rotation. Thesample stage can be adjusted by the skilled artisan to accommodate anydesired weight of sample.

Alternatively, the samples can be placed on a conveyor belt that travelsthrough the system along the axis of rotation of the arc 205.

The source/detector mechanism 203 rests on the same base 204 with thesample stage 202. The base needs to have high stability, i.e. givingheight variations of less than 0.003" under normal operation. Vibrationresistance can be achieved by putting the base on shock absorbing feet.The source/detector mechanism includes an arc 205 carrying an omegavehicle 206 and a 2theta vehicle 207. The arc 205 preferably extendsover more than 180° to accommodate the arc size of the vehicles 206 and207 when at near or past horizontal positions, though a smaller arccould also be used. The arc mechanism can be implemented as a (partial)circular precision track.

The track needs a guidance mechanism for the vehicles. Those of ordinaryskill in the art might devise any number of load bearing guidancemechanisms. Examples of guidance mechanisms include 1) an internal orexternal V groove for guiding V grooved journal wheels on the vehicles;or 2) a rail for guiding slide pads or flat journal wheels on vehicles.

Normally the track would have a drive mechanism for the vehicles, thoughconceivably the vehicles might contain their own drive mechanism. Thoseof ordinary skill in the art might devise any number of drivemechanisms. Examples of drive mechanisms include 1) a circular, spur, orworm gear for driving gear wheels on vehicles; or 2) a traction driverail for driving traction wheels on the vehicles. These drive mechanismsengage structures on the vehicles. Those structures are adjacent tothose portions of the guidance mechanisms which are positioned on thevehicles. The guidance mechanisms are described above.

Preferably, the track contains a position encoding mechanism. Those ofordinary skill in the art again might devise any number of positionencoding mechanisms. An example of a position encoding mechanism is anoptical or magnetic encoding strip.

The guiding, drive, and encoding mechanisms are preferably concentric toobtain improved precision and resolution.

The arc mechanism has a large radius, e.g. a source to sample distanceof more than 500 mm. As a result there is a large unobstructed spacearound the center of the diffractometer for large samples. The radius ofthe arc is preferably twice the maximal anticipated size of the sampleto allow full xy travel of the sample. Samples which are less than orequal to half the radius of the arc can assume arbitrary analysispositions. Larger samples are more constrained in their movements.

An x-ray source 208 rides on the omega vehicle 206. The source ispreferably high power, e.g. >1.5 kW. The x-ray detector 209 rides on the2theta vehicle 209. The detector and the source are preferably equippedwith vacuum pipes 210 and 211. The pipes have beryllium windows at eachend. These pipes preserve the x-ray energy which would otherwise bedissipated when traveling through the air to and from the sample. Thoseof ordinary skill in the art might devise other systems to preserve thex-ray energy such as x-ray lenses or x-ray mirrors. They would alleviatethe problem of dissipation by taking the divergent x-ray beam andturning it into a parallel beam. In this way, x-rays which wouldotherwise be lost could hit the sample and be useful, even though thepercentage of x-rays absorbed by the air would be the same as withoutsuch systems.

Normally, the vehicles will also include beam conditioners 220 and 221,also called x-ray optics, on both the primary and diffracted beam sides.These conditioners can include slits, which limit the divergence and/oracceptance widths, monochromating analyzer crystals, or other beamconditioners known to those of ordinary skill in the art.

Some beam conditioning devices can also be motorized to supportautomatic alignment or to switch between different devices.

The arc 205 is mounted on a support 212. The support may be integral tothe track or a separate structure which provides a pilot to the track,the latter being shown in the figures. The combined mechanism providesstability, accuracy, reproducibility and resolution equivalent to theprior art goniometer based x-ray diffractometer. The step size for thevehicles should be no more than 0.001° to give the desired resolution.The accuracy of the omega or 2theta positions after a movement of thevehicles must be better than 0.01°. The reproducibility requirement isthat every subsequent return to a same position must yield the sameposition to within less than 0.001°. Those of ordinary skill in the artcan devise mechanisms yielding various other tolerances as required forparticular measurements.

The support 212 in turn is mounted on chi tilt arm supports 213 and 214.A chi tilt drive mechanism 215 controls the chi angle of the arc 205.Counter balance weights 216 and 217 give stability. The design of thechi tilt mechanism uses the arc as a bypass of the chi-tilt axlemechanism to avoid interference with the sample. In other words the axleis split in two sections separated and held together by the arc.

FIG. 3 shows a side view of the system in accordance with the invention.Three possible chi angles of the system are shown. The vertical positionof the arc/support mechanism 301 is shown in solid lines. The tipped 302and horizontal 303 positions are shown in dotted lines. The counterweights move along with the arc/support system to positions 301', 302',and 303', corresponding to the positions 301, 302, and 303,respectively.

FIG. 4 shows an alternate embodiment of the invention in which the x-raysource and detector are slidably mounted to the vehicles 206 and 207.The lower positions 401 and 402 are suitable for smaller samples. Theraised positions 403 and 404, shown with dotted lines, are suitable forlarger samples. Alternatively, the vacuum pipes may be removed forlarger samples. To allow for such a possibility the vacuum pipes in FIG.2 can be removably mounted to the vehicles.

FIG. 5 shows another alternate embodiment. In this embodiment, thesource 501 and detectors 502 and 503 travel along separate, concentrictracks mounted on the same support. Since such tracks are completelymechanically separate, the movement of the source and detector vehicleswould still be completely independent. Having 2 detectors allows for agreater variety of measurements than a single vehicle allows.

We claim:
 1. An x-ray diffractometer comprising:a. an x-ray source; b.an x-ray detector; c. a sample stage suitable for accommodating a sampleof at least 25 cm; and actuation means for moving the x-ray source,x-ray detector, and the sample stage according to six degrees of freedomwith sufficient stability, precision, and reproducibility so that thex-ray diffractometer functions for samples of at least 25 cm.
 2. Thediffractometer of claim 1 wherein none of the motions along the sixdegrees of freedom results in tilting of the sample stage, whereby thesample need not be attached to the sample stage.
 3. The diffractometerof claim 2 wherein the at least one arc-shaped track has a radiusgreater than 500 mm.
 4. The diffractometer of claim 2 further comprisinga chi tilt mechanism for tilting the arc-shaped track.
 5. Thediffractometer of claim 1 further comprising a base giving a stabilityresulting in height variations of less than 0.003".
 6. Thediffractometer of claim 1, wherein a reproducibility of movement of thecomponents is such that a return to a given position gives a sameposition to within 0.001°.
 7. The diffractometer of claim 1 furthercomprisingan arc shaped track; an omega vehicle for transporting thex-ray source along the arc-shaped track; and a 2 theta vehicle fortransporting the x-ray detector along the arc-shaped track,independently from the omega vehicle.
 8. The diffractometer of claim 7further comprising first and second vacuum pipes mounted on the x-raysource and x-ray detector, respectively.
 9. The diffractometer of claim7, wherein the x-ray source and the x-ray detector are slidably mountedon their respective vehicles so that the source and detector are movablein a radial direction with respect to an axis of the arc of the at leastone arc-shaped track.
 10. The diffractometer of claim 7, wherein theradius of the arc is at least twice the anticipated size of the sample.11. The diffractometer of claim 7, wherein a step size of the vehiclesis no more than 0.001°.
 12. The diffractometer of claim 7, wherein theaccuracy of the omega and 2 theta positions after movement of thevehicles is better than 0.01°.
 13. An x-ray diffractometer comprising:a.a first portion comprising:an arc having a first axis; ii. a sourcevehicle for transporting an x-ray source about the arc; iii. a detectorvehicle for transporting an x-ray detector about the arc, independentlyfrom the source vehicle; and iv. a chi-tilt mechanism for tilting thearc about a second axis which is perpendicular to the first axis; and b.a sample stage having a top surface, the sample stage being isactuatable according to at least third, fourth, fifth, and sixth axes,with the top surface staying horizontal, the third axis being a verticleaxis of rotation perpendicular to both the first and second axes, thefourth, fifth and sixth axes being x, y, and z type axes,whereby thediffractometer has six degrees of freedom and the sample need not beattached to the sample stage.